1. Field of the Invention
The present invention relates to a pipe joint, and more particularly, to a pipe joint having threaded sleeve and spigot elements which absorb the pipe joint load.
2. Description of the Prior Art
Pipe joints connect tubular elements, for example pipelines, which deliver, in particular, pressurized fluids, such as for example gas or oil. In the search for oil or gas, these pipes are used as casing pipes or delivery pipes.
The pipe connections in question here are usually made using threads, an external thread on a spigot element and an internal thread in a sleeve element, which are screwed together. As far as the definitive positioning after the screwing-together operation is concerned, a distinction is made between pipe joints with an alignment marking and pipes joints which have an additional abutment element such as an inner shoulder or an outer shoulder. This shoulder element may additionally function as a seal. The shoulder essentially comprises annular surfaces which are arranged, for example, at the free end of the spigot element and on the inner side of the sleeve element and are pressed firmly one upon the other by the screwing-together operation. In the case of an inner shoulder, the spigot element is compressed between the shoulder and the thread and the sleeve element is expanded between the shoulder and the thread. In the case of an outer shoulder, the spigot element is expanded between the shoulder and the thread and the sleeve element is compressed between the shoulder and the thread. The sleeve and spigot stressing caused by the contact pressure in the shoulder is predominantly absorbed only by the small number of teeth at the end of the thread which are located adjacent to the shoulder surfaces. The remaining thread teeth are only subjected to an extremely small degree of loading. Accordingly, the loaded thread teeth are highly stressed. This high stressing in the thread is increased by external loading on the connection, in particular by axial tension and/or compression, but also by internal and external pressure and by bending, with the result that the overall stressing can reach a value which is greater than the permissible yield point. Axial tension can eliminate the prestressing in the abutment, and thus the positioning and sealing action thereof, and axial compression can result in plastic deformation in the shoulder region and, in the case of subsequent tensile loading, can render the positioning and feeling functions ineffective.
To distribute the forces acting on the sealing surfaces, i.e. abutment surfaces as reaction forces over the largest possible number of thread teeth, German Patent number DE 34 31 808 A1 discloses a specially designed pipe joint. In this reference, the pipe joint has an intermediate threaded section between the ends of the spigot thread and sleeve thread, in which the two elements have the same lead. In the end sections, the leads of the spigot thread and sleeve thread differ, to be precise in dependence on the position of the abutment:
If the annular sealing surfaces, i.e. abutment surfaces, are located on the inside of the pipe, then the thread of the spigot element has a greater lead than the thread of the sleeve element; if the sealing surface is located on the outside of the pipe, the opposite is true. The proposed arrangement is intended to make it possible for the reaction force originating from the abutment to be transmitted uniformly over the thread teeth. However, loading, such as axial compression and bending, is further introduced largely into the abutment, which is already prestressed to a considerable extent by the elements being screwed together. The pipe joint disclosed in this reference can also be used for a pipe joint with a multiple-start thread.